Optical fiber ribbon cables are well known for the transmission of optical signals, such as is described in U.S. Pat. Nos. 3,920,432, 4,289,558 and 4,980,007. As the demands on communication media continue to increase, the advantages of using optical cables for transmission of signals and for interconnecting local devices, continue to grow. With this growth comes the need to connect ribbon cables to a multiplicity of devices.
Optoelectronic device receptacles are necessary components when such ribbon cables are to be connected to equipment, circuit boards and the like. These receptacles provide the interface between the fiber optic cable and the semiconductor optoelectronic devices that generate or detect optical signals. They provide the location for conversion between optical and electrical signals. The receptacle serves as a housing or package for the optoelectronic device, protecting it from the environment. If the receptacle is connectorized, it provides a releasable mechanical mating mechanism that accepts the connector on the optical fiber cable. The receptacle provides optical coupling between the optoelectronic device and the fiber cable, and typically provides means for making electrical contact to the device and thermal contact for cooling the device.
Connecting optoelectronic components such as photodiodes, LED's or lasers to the ends of fiber optic cables has in the past been a difficult and expensive task. This is due to the fact the light emitting and detecting areas on the optoelectronic devices, and the optical cores of the optical fibers, are very small in size. Therefore, precision alignment between devices and fibers is required. An alignment tolerance of about 10 .mu.m is required for good optical coupling in a system based on 62.5 .mu.m core multimode fiber, and an alignment tolerance of about 1 .mu.m is required for 8 .mu.m core single mode fiber. Maintaining precision alignment is an issue not only in the initial placement of the optoelectronic component in the receptacle, but also in the design of the receptacle to maintain alignment throughout the operating temperature range and in the presence of mechanical forces resulting from vibration, connection or pulling of the cable.
Optical coupling between an optoelectronic device and an optical fiber in the connectorized cable can be obtained in a variety of ways. In principle, a cleaved or lensed fiber end, held in the cable connector, could be positioned in close proximity and alignment to the optically active area of the semiconductor optoelectronic device. However, having an optoelectronic device not protected from the surrounding environment when the connector is not inserted in the receptacle is not practical. An unprotected optoelectronic device is unacceptable with the devices currently in use because they are degraded by the contact with open air, and can be easily damaged by mechanical contact. Therefore, in known receptacles, devices are typically enclosed in a hermetically-sealed windowed package (e.g. a standard TO can which is known to those skilled in the art) before being mounted in the receptacle, and optical coupling occurs through a flat or lensed window mounted in the package wall. Obtaining efficient optical coupling between the hermetically-packaged device and the fiber may require additional lenses outside the hermetic package. Of course, all of the components in the optical path must be precisely aligned in order to achieve maximum coupling.
The above approach, while useable, has several disadvantages. First of all, it does not make good use of the precision that is built into optoelectronic devices. As a result of the photolithographic and etching processes that are used to fabricate such devices, submicron physical features are possible. Although these features could in principle be used to aid in alignment to fibers, this is not possible in the typical packaging approach discussed above. This is due to the fact that when these devices are mounted in standard hermetically sealed packages, they are not precisely placed in the packages; therefore the package body cannot be used as a reference surface to position the package precisely in the receptacle. Having thus lost the precision originally built into the device, the active area of the device must be optically aligned to the fiber actively. That is, the device must be energized and robotically moved into the position of maximum optical coupling. This type of active alignment process is slow and expensive.
Another disadvantage of the traditional approach is that it is not well-suited to fiber ribbon systems. A key reason for this is that the spacing between optical fibers in a ribbon cable is small, typically 250 microns. Therefore, due to size constraints, individual TO-style packages cannot be used. Packaging an array of devices in a single windowed package is also impractical. This is due to the difficulty associated with producing either a single optical element, or an array of micro-optical lenses, which can couple an array of fibers (at 250 micron pitch) to an array of devices, while maintaining high efficiency and low channel-to-channel optical crosstalk.
Several solutions have been proposed for optically coupling an array of optoelectronic devices to an optical fiber ribbon. The existing techniques generally fall into two categories. The first technique involves passive alignment of an alignment block to a substrate surface and therefore is generally easier to manufacture, but is more imprecise and therefore not adequate for achieving reproducible high-efficiency optical coupling between devices and optical fiber, especially in the case of single-mode fiber. The second technique generally involves active alignment of an optoelectronic component to a surface on an alignment block and therefore is more difficult and expensive to manufacture, but is more precise and effective in achieving reproducible, low-loss optical coupling.
Examples of the first technique are shown in PCT Appl. No. PCT/US94/05749 to Swirhun, et al. and in Bona, G. L. et al., Parallel Optical Links With 50 .mu.m Ribbon Fibers: Laser Array Concepts and Fiber Skew Analysis, Proceedings of the 20th European Conference on Optical Communication, 1994, Vol. 2, pp. 829-832. In each of these systems, the optoelectronic component (generally an array of optoelectronic elements) is mounted on a substrate surface and electrically connected thereto. The substrate surface is provided with one or more mechanical alignment features (e.g., holes, slots or channels) which mate with alignment features of an alignment block carrying a plurality of fiber optic stubs. In this way, there is a passive, mechanical alignment of the fiber optic stubs in the alignment block with the optoelectronic elements of the optoelectronic component.
While this technique is well-suited for lower performance interconnections with fibers having large core sizes (e.g. 62.5 .mu.m core standard multi-mode fiber), the inherent mechanical tolerances of the mechanical alignment features tend to limit the use of these techniques in higher performance, small-core fiber interconnections. In addition, the need to mount the optoelectronic device on a substrate surface imposes difficulties in producing an optoelectronic receptacle which can be connectorized in a plane of orientation parallel to the plane of the circuit board on which the device is to be mounted.
The Bona reference specifically discloses a parallel optical link for butt-coupling an optical component. Alignment is accomplished via alignment pins. While alignment pins are satisfactory for general alignment, they have significant disadvantages when precision alignment is required, especially for wide multifiber interfaces. In particular, the alignment of two connector alignment blocks via two pins suffers from the problem that the system is mechanically overconstrained. That is, the pins may prevent the faces of the two connector blocks from coming into intimate contact if the pins are not perfectly perpendicular to the block face. The Bona reference also discloses an optical link having optical fibers extending only a portion of the way through the optical link.
Examples of the second technique are shown in U.S. Pat. Nos. 5,359,686 to Galloway et al., 5,271,083 to Lebby, et al. and 5,265,184 to Lebby et al.. In these systems, the optoelectronic component is mounted directly on the alignment block, typically by use of a transparent contact adhesive or by solder bump bonding. The alignment block in each case is comprised of a plurality of molded waveguides encased in an injection-molded, plastic alignment block. In all of these patents, the plastic alignment block also includes electrical contacts on the surface to which the optoelectronic component is to be mounted for providing electrical power to the optoelectronic component. The electrical contacts are integrated into the plastic alignment block and connected to a leadframe extending therefrom.
The use of molded plastic waveguides or a molded plastic alignment block containing both the waveguides and the electrical contacts is disadvantageous for a number of reasons. First, the molded waveguides, while optically tuned to the particular optical fiber of a fiber optic ribbon, will be of an optically different material and there will necessarily be optical loss by virtue of this difference in materials. Second, the alignment block will be limited by the thermal and mechanical properties of the molded plastic (e.g. thermal expansion and thermal conductivity), which in some cases are not sufficient for higher performance applications. Finally, the integrated electrical contacts tend to create noise and interference at high operational frequencies, which also limits the application of this technique in high speed performance environments.
While existing techniques for creating receptacles for optoelectronic components in order to connectorize the optoelectronic component to an optical fiber ribbon have been effective for lower performance it would be desirable to provide an optoelectronic device receptacle and method of making the same which overcame the disadvantages of the existing techniques and which was more cost effective and easier to manufacture.